1. Field of the Invention
The present invention relates to a semiconductor laser device mounted on a heat sink and a manufacturing method thereof.
2. Description of the Background Art
A high power semiconductor laser device is indispensable as a light source for a recordable optical disc system and must have high reliability. One of the reasons why increase in the power of the semiconductor laser device has been restricted is COD (Catastrophic Optical Damage). The COD is believed to occur in the following cycle.
When current is injected to a facet of a cavity having a surface state in a high density, non-radiative recombination is caused through the surface state, and heat is generated. The generated heat reduces the energy gap at the facet portion, so that light is absorbed, which increases the heat generation. As this cycle is repeated, the temperature at the facet increases, and the crystal melts.
As a method of restricting the COD, the use of a current blocking region near the facet and a window structure by Zn diffusion are disclosed in ELECTRONICS LETTERS, Vol. 33, No. 12, pp. 1084-1086, 1997 and IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 29, No. 6, pp. 1874-1879, 1993.
FIG. 10 is a partly cut away, perspective view of a conventional semiconductor laser device having a current blocking region near the facet. FIG. 11 is a partly cut away, perspective view of a conventional semiconductor laser device having a window structure.
In FIGS. 10 and 11, an n-GaInP buffer layer 32, an n-AlGaInP cladding layer 33, a quantum well active layer 34 and a p-AlGaInP first cladding layer 35 are formed in this order on an n-GaAs substrate 31.
In a stripe-shaped region on the p-AlGaIn first cladding layer 35, a p-AlGaInP second cladding layer 36 and a p-GaInP contact layer 37 are formed in this order. These p-AlGaInP second cladding layer 36 and p-GaInP contact layer 37 form a ridge portion R.
An n-GaAs current blocking layer 38 is formed on the p-AlGaInP first cladding layer 35 and both sides of the ridge portion R. The n-GaAs current blocking layer 38 is also formed on regions at the upper surface of the ridge portion R in the vicinity of both facets.
A p-GaAs cap layer 39 is formed on the n-GaAs current blocking layer 38 and the ridge portion R.
Thus, a laser structure 60 of the plurality of layers 32 to 39 is formed on the n-GaAs substrate 31. On the back surface of the n-GaAs substrate 31, an n-electrode 42 is formed. On the upper surface of the laser structure 60, a p electrode (not shown) is formed.
As described above, since the n-GaAs current blocking layer 38 is formed in the regions at the upper surface of the ridge portion R in the vicinity of the facets of the cavity, current is not injected into the regions in the vicinity of the facets. Therefore, the COD is restrained.
Particularly in the semiconductor laser device in FIG. 11, a Zn diffusion region 43 by Zn diffusion is provided in the region in the vicinity of a facet of the quantum well active layer 34. Thus, a window structure allowing a wide band gap is formed in the region of the quantum well active layer 34 in the vicinity of a facet. As a result, there is no light absorption in the vicinity of the facet, and the COD is more restrained.
FIG. 12 is a schematic perspective overview of a conventional high power semiconductor laser device having the laser structure in FIG. 10 or 11. FIG. 13 is a schematic plan view of the semiconductor laser device in FIG. 12. FIG. 14 is a schematic sectional view of the semiconductor laser device in FIG. 12 taken along the length of the cavity.
In the laser structure 60 shown in FIGS. 10 and 11, the n-GaAs current blocking layer 38 is formed only in the regions in the vicinity of the facets on the upper surface of the ridge portion R, and raised portions 50 are formed at the p-GaAs cap layer 39 in the regions in the vicinity of the facets.
Furthermore, as shown in FIGS. 12 to 14, a p-electrode 41 is formed on the upper surface of the semiconductor laser structure 60. Raised regions 51 are formed at the p-electrode 41 because of the raised portions 50. The emitting point 53 of a laser beam is positioned at a facet of the quantum well active layer 34 under the raised portion 50 and the raised region 51.
FIG. 15 is a schematic sectional view of the semiconductor laser device in FIG. 12 provided on a sub-mount taken along the length of a cavity. FIG. 16 is a schematic front view of the semiconductor laser device in FIG. 12 provided on a sub-mount.
As shown in FIGS. 15 and 16, when the semiconductor laser device 300 is mounted junction down on the upper surface of the sub-mount 400 as the p-electrode 41 faces downward, only the raised portions 51 of the p-electrode 41 are in contact with the upper surface of the sub-mount 400. Therefore, at the time of die-bonding or wire-bonding, great stress is locally applied to the portions in the vicinity of the facets of the semiconductor laser device 300. Since the area of contact between the p-electrode 41 and the sub-mount 400 is limited, good heat-radiation characteristic does not result and the adhesion intensity is low. The semiconductor laser device 300 could be mounted tilted on the sub-mount 400. As a result, the reliability of the semiconductor laser device 300 is lowered.
It is an object of the present invention to provide a highly reliable, high power semiconductor laser device having a raised portion on its upper surface and a method of manufacturing thereof.
A semiconductor laser device according to one aspect of the present invention comprises a substrate, a laser structure formed on the substrate and including an active layer forming a cavity, and an electrode layer formed on the laser structure, the laser structure has a raised portion on its upper surface, the electrode layer has a first film thickness of zero or more in a region on the raised portion and a second film thickness larger than the first film thickness in the region excluding the raised portion.
Here, the first thickness may be zero, in other words, an electrode layer does not have to be formed at the raised portion.
In the semiconductor laser device, a laser structure including an active layer is formed on the substrate, and an electrode layer is formed on the laser structure. The electrode layer has a thickness larger than that in the raised portion in the region excluding the raised portion of the laser structure. Therefore, when the semiconductor laser device is mounted junction down on the upper surface of the heat sink as the electrode layer faces downward, the electrode layer is in contact with the heat sink in a large area. As a result, stress is not applied upon a particular part of the semiconductor laser device, but scattered in the whole of the semiconductor laser device and reduced. The contacting area between the electrode layer and the heat sink increases, so that the heat-radiation characteristic is improved, and the adhesion intensity is improved as well. In addition, the semiconductor laser device can be mounted stably almost without being tilted on the heat sink. As a result, the semiconductor laser device has higher reliability.
The second film thickness is preferably at least the sum of the height of the raised portion and the first film thickness. Thus, when the semiconductor laser device is provided on the upper surface of the heat sink as the electrode layer faces downward, the entire upper surface of the electrode layer is in contact with the upper surface of the heat sink. Therefore, stress is not applied upon a particular part of the semiconductor laser device, but sufficiently scattered in the whole of the semiconductor laser device and reduced. The contacting area between the second electrode and the heat sink sufficiently increases, so that the heat-radiation characteristic is more improved, and the adhesion intensity is more improved as well. In addition, the semiconductor laser device can be mounted more stably without being tilted on the heat sink. As a result, the semiconductor laser has even higher reliability.
The electrode layer may include a first electrode formed on the upper surface of the laser structure to cover at least a part of the raised portion and a second electrode formed on the first electrode excluding a raised region formed in the first electrode because of the raised portion.
In this case, the raised region is formed in the first electrode because of the raised portion in the laser structure. Thus, the second electrode is formed in the region excluding the raised region of the first electrode. Thus, when the semiconductor laser device is mounted junction down on the upper surface of the heat sink as the second electrode faces downward, a large area on the upper surface of the second electrode is in contact with the upper surface of the heat sink.
The first electrode and the second electrode may be formed of different materials or the same material.
The laser structure may include a cladding layer of a first conductivity type, an active layer, and a cladding layer of a second conductivity type in this order, the cladding layer of the second conductivity type may have a flat portion, and a ridge portion formed in a striped region on the flat portion, the laser structure may further include a current blocking layer of the first conductivity type formed on the flat portion on both sides of the ridge portion, on sides of the ridge portion and in a region on the upper surface of the ridge portion on the side of a facet of the cavity, and the raised portion may be formed because of a part of the current blocking layer formed in the region on the upper surface of the ridge portion on the facet side.
In this case, the current blocking layer of the first conductivity type is formed on the flat portion on both sides of the ridge portion, on sides of the ridge portion and in a region on the upper surface of the ridge portion on the side of a facet of the cavity, and therefore current injected from the electrode layer is injected to the ridge portion excluding the region on the side of a facet of the cavity.
Thus, since current is not injected to the region in the vicinity of the facet of the cavity, the COD is restrained. As a result, a high power semiconductor laser device is formed.
The raised portion may include a pair of raised parts formed on the sides of both facets of the cavity.
The active layer may have a quantum well structure, and the region of the active layer on the side of a facet of the cavity may have a band gap larger than in the other region of the active layer.
In this case, a window structure having a large band gap is formed in the region of the active layer in the vicinity of the facet. As a result, there is no light absorption in the vicinity of the facet of the cavity, so that the COD is more restrained. Therefore, a higher output semiconductor laser device is formed.
The region of the active layer on the side of a facet of the cavity may have a band gap larger than in the other region of the active layer because of impurity introduction.
In this case, the quantum well structure is disordered by introducing an impurity in the vicinity of the facet of the active layer, and a window structure having a large band gap is formed. As a result, there is no light absorption in the vicinity of the facet of the cavity, so that the COD is more restrained and a higher power semiconductor laser device is formed.
The active layer may have an ordered structure, a so-called natural super lattice, and the natural super lattice may be disordered by introducing an impurity only to the vicinity of the facet of the active layer to form a window structure.
The semiconductor laser device may further include a heat sink mounted on the electrode layer. In this case, the semiconductor laser device is securely mounted junction down on the upper surface of the heat sink as the electrode layer faces downward.
The electrode layer may have a first film thickness larger than zero in a region on the raised portion and a second film thickness larger than the first film thickness in the region excluding the raised portion. In this case, a relatively thin electrode layer is formed on the raised portion, and a relatively thick electrode layer is formed on the region excluding the raised portion.
The electrode layer may have a first film thickness of zero in the region on the raised portion, and a second thickness larger than zero in the region excluding the raised portion. In this case, no electrode layer is formed on the raised portion and an electrode layer is formed on the region excluding the raised portion.
A method of manufacturing a semiconductor laser device according to another aspect of the present invention comprises the steps of forming on a substrate a laser structure including an active layer forming a cavity and a raised portion on an upper surface of the laser structure, and forming on the laser structure an electrode layer having a first film thickness of zero or more in a region on the raised portion and a second film thickness larger than the first film thickness in the region excluding the raised portion.
Here, the first film thickness may be zero, in other words, an electrode layer does not have to be formed in the raised portion.
According to the method of manufacturing a semiconductor laser device, a laser structure including an active layer is formed on the substrate, and an electrode layer is formed on the laser structure. The electrode layer has a thickness larger in the region excluding the raised portion of the laser structure than in the region on the raised portion. Thus, when the semiconductor laser device is mounted junction down on the upper surface of the heat sink as the electrode layer faces downward, the electrode layer is in contact with the heat sink in a large area. As a result, stress is not applied upon a particular part of the semiconductor laser device, but scattered in the whole of the semiconductor laser device and reduced. The contacting area between the electrode layer and the heat sink increases, so that the heat-radiation characteristic is improved, and the adhesion intensity is improved as well. In addition, the semiconductor laser device can be mounted stably almost without being tilted on the heat sink. As a result, the semiconductor laser device has higher reliability.
The second film thickness is preferably at least the sum of the height of the raised portion and the first film thickness. Thus, when the semiconductor laser device is mounted junction down on the upper surface of the heat sink as the electrode layer faces downward, the entire upper surface of the electrode layer is in contact with the upper surface of the heat sink. As a result, stress is not applied upon a particular part of the semiconductor laser device, but sufficiently scattered in the whole of the semiconductor laser device and reduced. The contacting area between the electrode layer and the heat sink sufficiently increases, so that the heat-radiation characteristic is more improved, and the adhesion intensity is more improved as well. In addition, the semiconductor laser device can be mounted stably without being tilted on the heat sink. As a result, the semiconductor laser device has higher reliability.
The step of forming the electrode layer may include the steps of forming a first electrode to cover at least a part of the raised portion on the upper surface of the laser structure, and forming a second electrode on the first electrode excluding a raised region formed in the first electrode because of the raised portion.
In this case, the raised region is formed in the first electrode because of the raised portion of the laser structure. Thus, the second electrode is formed in the region excluding the raised region of the first electrode. As a result, when the semiconductor laser device is mounted junction down on the upper surface of the heat sink as the second electrode faces downward, a large area of the upper surface of the second electrode is in contact with the upper surface of the heat sink.
The step of forming the laser structure may include the steps of forming a cladding layer of a first conductivity type, the active layer, and a cladding layer of a second conductivity type having a flat portion and a ridge portion formed in a striped region on the flat portion and forming a current blocking layer of the first conductivity type on the flat portion on both sides of the ridge portion, on sides of the ridge portion, and in a region on the upper surface of the ridge portion on the facet side of the cavity, and the raised portion may be formed because of a part of the current blocking layer formed in the region on the upper surface of the ridge portion on the side of a facet of the cavity.
In this case, the current blocking layer of the first conductivity type is formed on the flat portion on both sides of the ridge portion, on sides of the ridge portion and in a region on the upper surface of the ridge portion on the side of a facet of the cavity, and therefore current injected from the electrode layer is injected to the ridge portion excluding the region on the side of a facet of the cavity.
Current is thus not injected into the region in the vicinity of the facet of the cavity. Therefore, the COD is restrained. As a result, a high power semiconductor laser device is formed.
The method of manufacturing a semiconductor laser device may further include the step of mounting a heat sink on the electrode layer.
The raised portion may include a pair of raised parts formed on the sides of both facets of the cavity.
The active layer may have a quantum well structure, and the region of the active layer on the side of a facet of the cavity may have a band gap larger than in the other region of the active layer.
In this case, a window structure having a large band gap is formed in a region of the active layer in the vicinity of the facet. Therefore, there is no light absorption in the vicinity of the facet of the cavity. Consequently, a higher power semiconductor laser device is formed.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.